Optimizing alignment in optical communication

Abstract

A method and system for maintaining an optimized point ahead angle (Θa) between an uplink beam (U 0 ) from a first terminal ( 10 ) to a second terminal ( 20 ), and a downlink beam (D 0 ) from the second terminal ( 20 ) to the first terminal ( 10 ). While transmitting data via a transmission channel (Tx) from the first terminal ( 10 ) to the second terminal ( 20 ), a directional variation (ΔΘ) is applied to an initial point ahead angle (Θ 0 ) for emitting the uplink beam (U 1 ,U 2 ) along different point ahead angles (Θ 1,Θ2 ). A respective intensity (I 1 ,I 2 ) is determined of a part of the uplink beam (U 1 ,U 2 ), received by the second terminal ( 20 ), emitted by the first terminal ( 10 ) along a respective point ahead angle (Θ 1, Θ2 ). The initial point ahead angle (Θ 0 ) is adjusted towards an optimized point ahead angle (Θa) based on the respective intensity (I 1 ,I 2 ) as function of the different point ahead angles (Θ 1,Θ2 ).

The invention claimed is: 
     
       1. A method for maintaining a point ahead angle between an uplink beam from a first terminal to a second terminal, and a downlink beam from the second terminal to the first terminal, the method comprising:
 providing a communication link for optical communication between the first terminal and the second terminal, wherein the communication link comprises:
 a reception channel through which the first terminal receives data via the downlink beam from the second terminal, and 
 a transmission channel through which the first terminal transmits data via the uplink beam to the second terminal along an initial point ahead angle with respect to the downlink beam; 
 
 applying, while transmitting data via the transmission channel from the first terminal to the second terminal, a directional variation to the initial point ahead angle for emitting the uplink beam along different point ahead angles at different instances of time; 
 determining, at the second terminal, a respective intensity of a part of the uplink beam received by the second terminal that is emitted by the first terminal along a respective point ahead angle; 
 communicating an indication of the respective intensity, determined by the second terminal, to the first terminal via the reception channel and/or communicating an indication of the respective point ahead angle, used by the first terminal in emitting the uplink beam, to the second terminal via the transmission channel; and 
 adjusting, in the first terminal, the initial point ahead angle towards a point ahead angle based on the respective intensity, determined in the second terminal, as a function of the different point ahead angles at which the uplink beam was emitted by the first terminal, 
 wherein the emitting the uplink beam along the different point ahead angles comprises:
 emitting, by the first terminal, the uplink beam as a first beam along a first point ahead angle with respect to the downlink beam; 
 determining, by the second terminal, a first intensity of a part of the first beam received by the second terminal; 
 emitting, by the first terminal, the uplink beam as a second beam along a second point ahead angle with respect to the downlink beam, wherein the second point ahead angle is different from the first point ahead angle; 
 determining, by the second terminal, a second intensity of a part of the second beam received by the second terminal; and 
 adjusting, by the first terminal, the initial point ahead angle based on a difference between the first intensity and second intensity. 
 
 
     
     
       2. The method of  claim 1 , wherein the initial point ahead angle is adjusted in a direction of highest intensity. 
     
     
       3. The method of  claim 1 , wherein a direction of the point ahead angle is continuously varied as a function of time, wherein an average or center of the varying point ahead angle is determined based on respective intensities varying as a result of the applied directional variation. 
     
     
       4. The method of  claim 1 , wherein the directional variation is applied by periodically tilting a point ahead mirror or fast steering mirror back and forth. 
     
     
       5. A method for maintaining a point ahead angle between an uplink beam from a first terminal to a second terminal, and a downlink beam from the second terminal to the first terminal, the method comprising:
 providing a communication link for optical communication between the first terminal and the second terminal, wherein the communication link comprises:
 a reception channel through which the first terminal receives data via the downlink beam from the second terminal, and 
 a transmission channel through which the first terminal transmits data via the uplink beam to the second terminal along an initial point ahead angle with respect to the downlink beam; 
 
 applying, while transmitting data via the transmission channel from the first terminal to the second terminal, a directional variation to the initial point ahead angle for emitting the uplink beam along different point ahead angles at different instances of time; 
 determining, at the second terminal, a respective intensity of a part of the uplink beam received by the second terminal that is emitted by the first terminal along a respective point ahead angle; 
 communicating an indication of the respective intensity, determined by the second terminal, to the first terminal via the reception channel and/or communicating an indication of the respective point ahead angle, used by the first terminal in emitting the uplink beam, to the second terminal via the transmission channel; and 
 adjusting, in the first terminal, the initial point ahead angle towards a point ahead angle based on the respective intensity, determined in the second terminal, as a function of the different point ahead angles at which the uplink beam was emitted by the first terminal, 
 wherein a repeating directional variation is applied with a period shorter than a roundtrip time between the terminals. 
 
     
     
       6. The method of  claim 5 , wherein a first directional variation is applied in a direction along the point ahead angle and a second directional variation is applied in a direction transverse to the point ahead angle. 
     
     
       7. The method of  claim 5 , wherein the directional variation is configured to cause a variation in intensity of the part of the uplink beam received at the second terminal, wherein the variation in intensity is between one and twenty percent. 
     
     
       8. The method of  claim 1 , wherein a first directional variation is applied in a direction along the point ahead angle and a second directional variation is applied in a direction transverse to the point ahead angle. 
     
     
       9. The method of  claim 1 , wherein the first terminal sends synchronization information for determining which respective point ahead angle was used via the transmission channel to the second terminal. 
     
     
       10. A method for maintaining a point ahead angle between an uplink beam from a first terminal to a second terminal, and a downlink beam from the second terminal to the first terminal, the method comprising:
 providing a communication link for optical communication between the first terminal and the second terminal, wherein the communication link comprises:
 a reception channel through which the first terminal receives data via the downlink beam from the second terminal, and 
 a transmission channel through which the first terminal transmits data via the uplink beam to the second terminal along an initial point ahead angle with respect to the downlink beam; 
 
 applying, while transmitting data via the transmission channel from the first terminal to the second terminal, a directional variation to the initial point ahead angle for emitting the uplink beam along different point ahead angles at different instances of time; 
 determining, at the second terminal, a respective intensity of a part of the uplink beam received by the second terminal that is emitted by the first terminal along a respective point ahead angle; 
 communicating an indication of the respective intensity, determined by the second terminal, to the first terminal via the reception channel and/or communicating an indication of the respective point ahead angle, used by the first terminal in emitting the uplink beam, to the second terminal via the transmission channel; and 
 adjusting, in the first terminal, the initial point ahead angle towards a point ahead angle based on the respective intensity, determined in the second terminal, as a function of the different point ahead angles at which the uplink beam was emitted by the first terminal, 
 wherein an indication of the respective point ahead angle used by the first terminal for transmitting the respective uplink beam is sent by the first terminal via the transmission channel to the second terminal and the first terminal receives an indication of the respective intensity as a function of respective point ahead angle via the reception channel from the second terminal. 
 
     
     
       11. The method of  claim 1 , wherein a lateral variation in position of the uplink beam at the second terminal due to the directional variation is less than a full width half maximum of the uplink beam at the second terminal by at least a factor two. 
     
     
       12. The method of  claim 1 , wherein the directional variation is configured to cause a variation in intensity of the part of the uplink beam received at the second terminal, wherein the variation in intensity is between one and twenty percent. 
     
     
       13. The method of  claim 1 , wherein the initial point ahead angle for establishing the communication link is determined by a first alignment stage, wherein the first alignment stage prevents or interrupts the transmitting of data via the transmission and/or reception channel, wherein after the communication link is established a second alignment stage is used, wherein the point ahead angle is continually updated or maintained by the directional variation and resulting intensity measurements of the uplink beam while transmitting data via the transmission channel of the uplink beam, and receiving data via the reception channel of the downlink beam. 
     
     
       14. The method of  claim 13 , wherein the first alignment stage comprises relative alignment between the uplink beam and the downlink beam, wherein in the relative alignment, the uplink beam is redirected to a first sensor instead of being transmitted towards the second terminal, wherein the first sensor is used during communication with the second terminal for measuring the downlink beam, wherein the first sensor is a wavefront sensor, wherein the relative alignment comprises blocking an optical path of the communication link between the first terminal and the second terminal. 
     
     
       15. A first terminal configured to optically communicate with a second terminal, wherein the first terminal comprises:
 a light sensor arranged for measuring a downlink beam and configured to receive data via the downlink beam from the second terminal; 
 a light source arranged for generating an uplink beam and configured to transmit data via the uplink beam to the second terminal; 
 a point ahead mirror arranged in an optical path of the uplink beam and configured to variably set a point ahead angle determining a direction of transmission of the uplink beam with respect the downlink beam; and 
 a controller arranged to receive data via a reception channel, transmit data via a transmission channel, and control a direction of the point ahead mirror, wherein the controller is configured to:
 determine an initial point ahead angle for establishing a communication link with the second terminal; 
 apply a directional variation to the initial point ahead angle for emitting the uplink beam along different point ahead angles at different instances of time while transmitting data via the transmission channel from the first terminal to the second terminal; 
 reading data from the reception channel to determine a respective intensity of a part of the uplink beam, received by the second terminal, emitted by the first terminal along a respective point ahead angle; and 
 adjust the initial point ahead angle towards an optimized point ahead angle based on the respective intensity as function of the different point ahead angles, 
 wherein the controller is configured to control a direction of the point ahead mirror to apply a repeating directional variation with a period shorter than a roundtrip time between the terminals. 
 
 
     
     
       16. The first terminal of  claim 15  forming part of an earth based station, wherein the first terminal is configured to optically communicate with a satellite in orbit comprising the second terminal. 
     
     
       17. The first terminal of  claim 15 , wherein the controller is configured to control a direction of the point ahead mirror to apply the directional variation by periodically tilting the point ahead mirror back and forth. 
     
     
       18. The first terminal of  claim 15 , wherein the controller is configured to control a direction of the point ahead mirror to apply a first directional variation in direction along the point ahead angle and a second directional variation in a direction transverse to the point ahead angle. 
     
     
       19. The first terminal of  claim 15 , wherein the controller is configured to send synchronization information via the transmission channel for determining which respective point ahead angle was used via the transmission channel to the second terminal. 
     
     
       20. A first terminal configured to optically communicate with a second terminal, wherein the first terminal comprises:
 a light sensor arranged for measuring a downlink beam and configured to receive data via the downlink beam from the second terminal; 
 a light source arranged for generating an uplink beam and configured to transmit data via the uplink beam to the second terminal; 
 a point ahead mirror arranged in an optical path of the uplink beam and configured to variably set a point ahead angle determining a direction of transmission of the uplink beam with respect the downlink beam; and 
 a controller arranged to receive data via a reception channel, transmit data via a transmission channel, and control a direction of the point ahead mirror, wherein the controller is configured to:
 determine an initial point ahead angle for establishing a communication link with the second terminal; 
 apply a directional variation to the initial point ahead angle for emitting the uplink beam along different point ahead angles at different instances of time while transmitting data via the transmission channel from the first terminal to the second terminal; 
 reading data from the reception channel to determine a respective intensity of a part of the uplink beam, received by the second terminal, emitted by the first terminal along a respective point ahead angle; and 
 
 adjust the initial point ahead angle towards an optimized point ahead angle based on the respective intensity as function of the different point ahead angles, 
 wherein the controller is configured to: 
 send an indication of the respective point ahead angle used for transmitting the respective uplink beam via the transmission channel to the second terminal, and 
 receive an indication of the respective intensity as a function of respective point ahead angle via the reception channel from the second terminal.